Studies On Superconducting Nanowire Single-photon Detectors With High Efficiency And Large Active Area

Single photon detection is an important technology in quantum information and quantum communication fields. It is the premise for achieving the single quantum state to control, treatment and research. Superconducting nanowire single-photon detector （SNSPD） is a latest single photon detection, which has high system detector efficiency （SDE）, low dark counts rate （DCR）, high repetition rate and wide response spectrum. It is the hottest research area in recent years.With these years of fast development, the SNSPDs produced recently show great properties than the other existing single photon detectors. The SDE of SNSPD based on niobium nitride （NbN） thin film is over 70%, when the dark count rate is 100 Hz. The timing jitter is 50 ps and the maximum count rate is over 100 MHz for SNSPD based on NbN thin film. Due to the excellent properties, SNSPDs are widely applied in quantum key distribution, integrated circuit testing,biomedical fluorescent detection, fiber distributed sensing, characterization of quantum emitters, and time of flight depth imaging.This paper is based on theoretical calculation and experiment to research SNSPD.The primary results are given as follows:1. We designed and optimized high efficient SNSPD with optical resonant cavity, whose active area is 30 μm×30μm. Uniform and ultrathin NbN thin films （4-6 nm） were prepared by magnetron sputtering and studied. During the process of electron beam lithography, proximity effect was corrected by optimizing the design of exposure pattern and controlling the accelerating voltage. The exposure performances such as resolution ratio and sensitivity of several electron beam resists were studied systematically. The width, uniformity and steepness of nanowires at the large area were controlled finely by optimizing the thickness of resist, exposure dose, development and the etching process. SNSPDs have been prepared by using these uniform and steep nanowires. At the wavelength of 1550 nm, the system detection efficiency （SDE） was 72.4% with 100 cps DCR, and the highest SDE was 77.2%. The highest SDE appeared at the wavelength of 1650 nm, which is 81.2%. The performance indexes of our devices have reached the international advanced level.2. We designed the new composite optical structure of phase-grating and optical resonant cavity, and use it for producing SNSPDs. The optimizing of this structure was calculated by using FDTD solution. NbN SNSPDs with only a phase-grating structure were designed with the incident wavelength of 404 nm. The highest absorption is 95%, which appeared when the width of nanowires was 200 nm. Considering the producing process and the application field, we designed the working wavelength of our device is 850 nm. For exerting the broadband characteristics, the optical resonant cavity has been added in our design. By adjusting the parameters of the structure, the absorption of NbN nanowires can attain 72%, when the period of phase-grating was 1 nm, each period contained four 80 nm wide nanowires and the filling factor was 0.5. Strong absorption （over 40%） appeared around not only at the wavelength of 850 nm but also at wavelengths of 684,732,798,924,1256, and 1426 nm, which achieved the design of high SDE at broadband wavelengths. As only half area has been covered by NbN nanowires at one period, the length of nanowires was only half comparing to the traditional SNSPD, which would highly improved the detection rate and response time. By evaluating sufficiently the feasibility of process, SNSPDs with this composite optical structure can be achieved by existing microfabrication technique. It was so important for developing SNSPDs with wide bandbroad wavelength, high SDE and detection speed.3. We have produced NbN SNSPD with Nb₅N₆ buffer layer based on Si substrate, which have high transformation temperature and critical current density. We have used 5 nm thick NbN film grown on 30 nm Nb5N6 film to fabricate SNSPDs. The critical current attained 38 μA. The highest SDE was 13.95%, when the bias current was 36.1 μA and the DCR was 21 cps. At the same time, the dynamic inductance was only 297.8 nH, which was much lower than the dynamic inductance of tranditional NbN SNSPD. It meaned that the recovery time was greatly shortened and intrinsic detection speed has been highly improved.